The invention relates to a process for preparing partial hydrolysates of organometallic compounds and to a process for preparing partial hydrolysates of organometallic compounds or transition metal catalysts immobilized on inert support materials and also to the products prepared by this process.
Transition metal catalysts comprising transition metal compounds and organometallic compounds such as alkylaluminoxanes, in particular methylaluminoxane (MAO), are gaining increasing importance as essential constituents of a new generation of catalyst systems for preparing polyolefins (xe2x80x98single-site catalystsxe2x80x99). These new catalysts consist, as is already known from classical Ziegler-Natta catalysis, essentially of a transition metal compound as catalyst and an alkylaluminoxane as organoaluminium cocatalyst component. As transition metal compound, preference is given to using cyclopentadienyl, indenyl or fluorenyl derivatives of elements of group IVa of the Periodic Table of the chemical elements. In contrast to conventional Ziegler-Natta catalysts, such systems have a high activity and productivity and, in addition, not only the ability to control the product properties in a targeted manner as a function of the components used and the reaction conditions, but also open up a route to hitherto unknown polymer structures having very promising properties with a view to industrial applications.
Many publications concerned with the preparation of specific polyolefins by means of such catalyst systems have appeared in the literature. However, they virtually all have the disadvantage that a large excess of alkylaluminoxanes, based on the transition metal component, is necessary to achieve acceptable productivities (the ratio of aluminium in the form of the alkylaluminoxane to transition metal is usually about 1000:1xe2x80x94cf. W. Kaminsky et al., Polyhedron, Vol. 7, No. 22/23 (1988) 2375 ff). Due to the high price of alkylaluminoxanes and also due to additional polymer work-up steps (xe2x80x98deashing stepsxe2x80x99) required in some cases, polymer production on an industrial scale on the basis of such catalyst systems would frequently be uneconomical. In addition, the toluene which is frequently used as solvent for the formulation of alkylaluminoxanes, in particular methylaluminoxane, is becoming increasingly undesirable for reasons of storage stability of the formulations (strong tendency to form gel), and also with a view to the applications of the polyolefins which finally result.
A significant reduction in the amount of alkylaluminoxane required for a given amount of transition metal component can be achieved by applying alkylaluminoxane to inert support materials, preferably SiO2 (J. C. W. Chien, D. He, J. Polym. Science Part A, Polym. Chem., Vol. 29, 1603-1607 (1991)).
Furthermore, such supported materials have the advantage of being able to be separated off readily in the case of polymerizations in a condensed phase (preparation of high-purity polymers) or being able to be used as free-flowing powders in modern gas-phase processes, with the particle morphology of the polymer being able to be set directly by means of the particle shape of the support. Furthermore, alkylaluminoxanes immobilized on supports as dry powders are physically more stable than solutions having a comparable Al content. This applies particularly to methylaluminoxane which, as mentioned above, tends to form gel in toluene solution after a certain storage time.
A number of possible ways of immobilizing alkylaluminoxane on supports have been described in the literature.
EP 0 369 675 describes a process in which the immobilization of alkylaluminoxanes is achieved by reaction of an about 10% strength solution of trialkylaluminium in heptane with hydrated silica (8.7% by weight of H2O).
EP 0 442 725 describes a process in which the immobilization is effected by reaction of a toluene/water emulsion with an about 7% strength solution of trialkylaluminium in toluene in the presence of silica at temperatures of from xe2x88x9250xc2x0 C. to +80xc2x0 C.
EP-A-0 567 952 describes a supported polymerization catalyst comprising the reaction product of
A) a supported organoaluminium compound which is prepared by
(i) preparing a suspension of a support containing less than 3% by weight of water in a solution of at least one alkylaluminium compound under inert conditions and
(ii) hydrolysing the suspension by addition of water to the suspension and
B) a transition metal compound as catalyst.
A further alternative is offered by U.S. Pat. No. 5,026,797 by reaction of previously prepared alkylaluminoxane solutions with silica (predried at 600xc2x0 C.) at 60xc2x0 C. and subsequent washing-out of the alkylaluminoxane which has not been immobilized by means of toluene.
U.S. Pat. No. 4,921,825 describes a process for immobilizing alkylaluminoxanes by precipitation from toluene solutions by means of n-decane in the presence of silica.
Some of these processes are technically complicated since they employ, inter alia, low reaction temperatures at the beginning or multistage work-up processes and therefore suffer from losses in yield or the degrees of loading of the support with alkylaluminoxanes necessary for a high catalyst activity can often not be achieved. In addition, the particle morphology of the supported alkylaluminoxane and of the supported transition metal catalyst can be altered in an adverse and undesirable way. The filtration and drying steps sometimes lead to destruction of the carrier particle, resulting in formation of small fragments (xe2x80x98finesxe2x80x99) which can lead to reactor fouling in the polymerization. Furthermore, the alkylaluminoxanes prepared by these processes usually have a very broad distribution of the degree of oligomerization, which leads to inhomogeneous supported products.
EP 0 650 967 describes a process for the immobilization of alkylaluminoxanes on support materials, in which a dispersion of alkylaluminoxanes which has been prepared by addition of water to a solution of alkylaluminium compounds in hydrocarbons in a static mixer is immobilized on inert support materials. A disadvantage of this process is that the degree of oligomerization of the alkylaluminoxanes formed changes during the course of the process so that alkylaluminoxane oligomers having a broad distribution of the degree of oligomerization (oligomers having 1-20 units) are obtained (see also EP 0 623 624). The supported alkylaluminoxanes thus do not have the desired homogeneous nature.
It is therefore an object of the present invention to overcome the disadvantages of the prior art and to provide a process in which partial hydrolysates of organometallic compounds, in particular alkylaluminoxanes, immobilized on inert support materials can be obtained in high yield and homogeneity in a reproducible way. The new process should ensure that the degrees of loading can be varied within wide limits, that the particle morphology of the support is retained and that the products are finally obtained as free-flowing powder. The new process should give partial hydrolysis products of organometallic compounds which are immobilized on inert support materials and whose degree of oligomerization can be set to suit the specific process.
A further object of the invention is to provide a process for preparing partial hydrolysis products of organometallic compounds, in particular alkylaluminoxanes, in which partial hydrolysis products of organometallic compounds which have a degree of oligomerization which can be adjusted to suit the specific process are prepared simply, efficiently and in high yield.
The invention provides a process for preparing partial hydrolysates of organometallic compounds, in particular alkylaluminoxanes, immobilized on support materials, which is characterized in that the organometallic compounds and water are continuously introduced into a static mixer in the presence of hydrocarbons and the resulting reaction products are brought into contact with support materials. The resulting reaction products can be obtained in the form of solutions or dispersions, for example lyophilic dispersions in the sol state (cf. Rxc3x6mpp Chemie Lexikon, 9th edition, Georg Thieme Verlag Stuttgart, New York 1990, p. 2299 ff). Compared with the prior art, the process of the invention gives a higher space-time yield, higher homogeneity of the resulting partial hydrolysates of organometallic compounds immobilized on inert support materials and simpler control of the degree of loading. The novel process makes it possible to prepare homogeneous supported partial hydrolysis products of organometallic compounds having a degree of oligomerization and constitution which can be adjusted in a targeted manner, which is of great importance because of the influence of the degree of oligomerization of the partial hydrolysis products of organometallic compounds on their activity as cocatalysts in polymerization. Thus, employing the process of the invention results in a narrower particle size distribution of the polymer compared with the prior art. This is always observed when the molar mass is controlled in polyethylene production by introduction of hydrogen, but the distribution of the molar masses is now less pronouncedly bimodal. This result implies that the aluminoxane on which the catalyst is based has a narrower molar mass distribution.
The invention also provides a process for preparing transition metal catalysts immobilized on support materials, which is characterized in that one or more transition metal compounds are added in solid or dissolved form during or, if desired, after the above-described process. Compared with the prior art, the process of the invention gives a higher space-time yield, greater homogenity of the resulting transition metal catalysts immobilized on inert support materials and simpler control of the degree of loading.
The invention further provides a process for preparing partial hydrolysis products of organometallic compounds, in particular aluminoxanes, which is characterized in that the organometallic compounds and water are introduced continuously into a static mixer in the presence of hydrocarbons. Compared with the prior art, the process of the invention gives a higher space-time yield and the resulting partial hydrolysis products of organometallic compounds have a degree of oligomerization which can be adjusted in a targeted manner.
The invention further provides partial hydrolysis products of organometallic compounds immobilized on support materials, wich have been prepared by the process of the invention.
The invention also provides transition metal catalysts immobilized on support materials, prepared according to the process of the invention.
The invention further provides the partial hydrolysis products of organometallic compounds, in particular aluminoxanes, prepared by the process of the invention.
A further subject-matter of the invention is characterized by the claims.